The present invention relates to a method of planarizing trench-like structures, particularly those on semiconductor substrates, by filling in the trench-like structures.
In the manufacture of optical polymer components it is known to provide preferably V-shaped trench-like structures on semiconductor substrates or other components including integrated optical waveguides so as to permit the later coupling of optical fibers placed into the V-shaped structures in the correct position to light waveguides provided on the optical element. For this purpose, the V-shaped positioning trenches are etched in anisotropically in a known manner. An optimum coupling location between the optical fibers and the light waveguides is produced by filling the V-shaped positioning trenches and subsequently opening them again by excimer laser ablation in such a way that a surface is created which extends perpendicularly to the axial direction of the structures and forms an abutment.
Particularly in the case of polymer light waveguides, it is possible to perform a joint lazer ablation of waveguide end face and fiber guide structure.
To ensure the optimum coupling between fiber and chip, it is absolutely necessary to planarize the created trench-like microstructures. Since the semiconductor substrates equipped with the recessed structures must be coated with photolacquers or other thin films in order to be processed further lithographically, it is generally possible to apply a planar coating only by first filling the trench-like structures so they become flush with the substrate surface.
It is known, for example, if recesses in a range of up to about 10 .mu.m are involved, to repeatedly coat the substrate surface and back etch an applied polyimide film in an oxygen plasma. With every new polyimide coating, the deviation from the desired planar surface remaining after the preceding process step is reduced a little. Depending on the depth and shape of the structures, this requires several repeated cycles with relatively high processing expenditures. Moreover, it is a drawback that, for example, in connection with the mentioned V-shaped optical fiber guide structures, structure depths of about 100 .mu.m occur which cannot be planarized according to the prior art method.
Further, European Patent 0,178,500 (which corresponds to U.S. Pat. No. 4,568,601) discloses a method in which a photostructurable polymer is applied over the entire semiconductor wafer and is cross-linked only locally at the structures to be planarized. Non-cross-linked polyimide is then developed away so that it projects only at the structures to be planarized and can then be planarized at reduced expense, for example by overcoating with subsequent plasma etching. However, it is a drawback that, in this method for the directed cross-linking of the polymer at the structures to be planarized, the illumination must occur through a precisely fitting photomask. Moreover, because of the non-planar substrate surface, the method is not suitable for the planarization of large-area substrates since the polymer overcoating must be extremely uniform to ensure success of the isotropically attacking plasma etching step. Particularly in the case of very deep structures to be planarized, for example V-shaped optical fiber guide structures of a depth of about 100 .mu.m, this method cannot be employed due to the considerable polymerization shrinkage of the applied materials.